I. Field of the Invention
The present invention relates to radio communications. More particularly, the present invention relates to mobile communication in a code division multiple access system.
II. Description of the Related Art
Multiple access techniques are designed to make efficient use of the limited radio frequency spectrum. They allow users to access the same band of frequency without interfering with each other. Examples of such techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
CDMA wireless technology, governed by Electronic Industry Association/Telecommunication Industry Association Interim Specification-95 (IS-95), employs a spread spectrum technique for the transmission of information. A spread spectrum system uses a modulation technique that spreads the transmitted signal over a wide frequency band. This frequency band is typically substantially wider than the minimum bandwidth required to transmit the signal.
A form of frequency diversity is obtained by spreading the transmitted signal over a wide frequency range. Since only part of a signal is typically affected by a frequency selective fade, the remaining spectrum of the transmitted signal is unaffected. A receiver that receives the spread spectrum signal, therefore, is affected less by the fade condition than a receiver using other types of signals.
The spread spectrum technique is accomplished by modulating each base band data signal to be transmitted with a unique wide band spreading code. Using this technique, a signal having a bandwidth of only a few kilohertz can be spread over a bandwidth of more than a megahertz. Typical examples of spread spectrum techniques are found in M. K. Simon, Spread Spectrum Communications, Volume I, pp. 262-358.
In a CDMA-type radiotelephone system, multiple signals are transmitted simultaneously on the same frequency. A particular receiver determines which signal is intended for that receiver by the unique, orthogonal spreading code, referred to as a Walsh code, in each signal. The signals at that frequency, without the particular spreading code intended for that particular receiver, appear to be noise to that receiver and are ignored.
A generic CDMA transmitter and receiver are illustrated in FIGS. 1A and 1B respectively. The transmitter in FIG. 1A convolutionally encodes (100) an input data signal. The encoded signal is spread (105) by the Walsh code (106) such that the output of the spreading operation yields the symbol stream of the Walsh code representing "0" and the complement of the Walsh code representing "1". This output is then modulated (110).
The receiver of FIG. 1B demodulates (115) the received signal. The demodulated signal is then despread (120) using the same Walsh code (116). The de-spread signal is de-interleaved and input to the Viterbi decoder (125).
For each user of the CDMA system of FIG. 1, symbol-0 is represented by a Walsh code and symbol-i is represented by its complementary Walsh code. Since W.sub.i and W.sub.i are non-orthogonal to each other, the bit error rate performance of this scheme is solely determined by the forward error control coding. The forward error control coding scheme is well known in the art.
The demand for reduced error transmission has been on the rise in recent years. New wireless services have emerged recently, such as broad band multimedia communications, that require lower bit error rates. Multimedia applications may require bit error rates (BER) as low as 10.sup.-10.
IS-95 CDMA, however, cannot reduce interference or improve capacity any further. As observed in FIG. 1, the symbol stream to be transmitted is the Walsh code and its complement which are always non-orthogonal. Therefore, a conventional IS-95 CDMA receiver correlates with a pair of non-orthogonal signals. This translates into a poor BER at the output of the decoder in a multipath environment.
The typical BER that is offered by the forward error control coding mechanism is inadequate for satellite and broadband multimedia applications. There exists an unforeseen need for a more robust multiple access radio communications technique for broadband terrestrial and satellite communications. The technique should decrease the BER while not having a detrimental affect on capacity.